The present invention relates to a method of and device for monitoring and/or controlling welding current in the high-frequency resistance pressure welding of pipes with longitudinal seams by detecting the magnetic fields generated by the welding current with a sensor.
The invention is based on that described in U.S. Pat. No. 3,573,416, from which the detection of welding currents behind the welding point from outside by means of magnetic measurements in the high-frequency welding of pipes with longitudinal seams is known. Other documents, German Offenlegungsschrift No. 1 925 965 for example, disclose similar solutions.
The drawback of the known methods is interference from magnetic fields that derive from the power lines leading to the welding equipment and that cannot be adequately screened out at the conventional welding frequencies of about 400 kHz. Other magnetic fields, such as those deriving from inductive annealing equipment like pipe-seam annealing equipment that may be being operated in the vicinity, can also interfere. Especially disruptive to detection of the current that flows through the edges of the skelp and heats them up, and which is of sole interest in this context, is something called pipe back current, which results from conductive, and even more from inductive, current coupling.
A very specific means of detecting welding phenomena and processing them to control welding energy is known from German Offenlegungsschrift No. 2 826 986. The device is, however, extremely expensive.